Computed tomography scanning (CT scanning), also denoted computerized tomography or computed axial tomography (CAT), is a medical imaging method employing imaging by sectioning or 3D reconstruction. In the CT-scanning an X-ray source and an X-ray detector are arranged opposite one another on an arrangement that rotates around a patient. The X-ray source transmits radiation through the patient and the X-ray detector measures the attenuated radiation. The radiation is converted to an electrical signal, a computer processes these signals and the desired images can be provided.
An important improvement of the two-dimensional scanning was made with the introduction of the so-called spiral or helical scan. Instead of scanning the patient on a two-dimensional basis, the patient is scanned on a three-dimensional basis. In particular, the table on which the patient rests moves through the scanning field at a constant speed while the X-ray tube and X-ray detector rotates around the patient. Digital geometry processing is used to generate a three-dimensional image of the internals of an object from a large series of two-dimensional X-ray images taken around a single axis of rotation.
The X-ray arrangement is rotated one revolution, then moved a small step and rotated again. Alternatively, in the spiral scan the X-ray arrangement moves in a helical movement with a small pitch for each revolution. The speed of rotation is normally a few revolutions per second.